Often, critical or very sensitive measurements must be made in a laboratory or on the manufacturing floor of a building wherein precision table top equipment such as electron microscopes and other microprecision equipment or machines are used for research, manufacturing, and quality control. Vibrations would otherwise adversely affect such equipment and hence vibration isolation systems are incorporated to suppress the vibrations and shocks occurring in the area the measurements are to take place. The vibrations may be the result of the natural frequency of the surrounding structure or due to extraneous elements such as rotating machinery or even other man made vibrations which enter the structural skeleton of the building and are subsequently transmitted throughout the building. Vibrations will be transmitted to the precision equipment at various frequencies and the vibrations will have both vertical and horizontal components.
Air or gas springs incorporated into vibration isolation system mounts are known which include convoluted diaphragm seal and piston type springs as well as conventional bellows type airsprings. These types of vibration isolation systems adequately provide vibration attenuation in the axial (vertical) direction. Axial vibration isolation is achieved in general by lowering the stiffness of the system in the axial direction to the extent that the system will still support the table top, platform, or other surface on which the precision equipment is located.
The object of these systems is to reduce the natural frequency and the stiffness of the mount and thereby increase the efficiency of vibration isolation. In a convolution air seal piston type vibration isolator, the natural frequency is a function of the air volume, the load support interface area, and the static load pressure.
Unfortunately, however, most of these types of airsprings are inherently stiff in the horizontal direction. As a consequence, vibration isolation is significantly less efficient horizontally than for the vertical direction.
Accordingly, different techniques have been employed to improve horizontal vibration isolation in air or gas springs. For example, one notable improvement, U.S. Pat. No. 4,223,762, employs a floating member within the piston of a convoluted diaphragm seal arrangement. Inner floating balls that ride in a hardened ball race of the floating member reduce vibrations in a direction orthogonal to the axis of the airspring while the airspring itself reduces vibration in the direction along the axis of the airspring. Alternatively, the floating member may be connected to one end of the airspring cylinder. Other techniques to reduce horizontal vibration also employ various modification of the piston itself including adding cables or rods to a hollow piston (U.S. Pat. No. 3,784,146) or a concentric load supporting rod pivotably engaging the bottom of a well of the piston (U.S. Pat. No. 4,360,184) which permits gimbel like rotation of the piston for horizontal movement of the load. As can be appreciated, each of these devices used to isolate the horizontal component of vibrations transmitted to the air piston assembly require substantial mechanical structure and intricate and generally close tolerance or precision fabrication techniques. One other prior technique employs a multilaminated rubber and stabilizing plate assembly integrally connected to a bellows type airspring (U.S. Pat. No. 5,018,701). Because the focus of this teaching is for earthquake vibration isolation of a floor structure, vertical guide posts are used to restrain displacement of the airspring only in the vertical direction. Accordingly, when used in a load support system with multiple supports, the rubber laminate assembly is only displaceable in the vertical or horizontal direction. Unfortunately, this configuration does not allow the airspring assembly itself to be displaced other than directly in compression in the vertical direction. That is, the airspring can not be deflected horizontally nor can it rock or pivot or rotate along or about its vertical axis. Thus, the horizontal stiffness at the load support interface is governed exclusively by the horizontal shear stiffness of the rubber laminate assembly. Also, like the other art discussed above, substantial structure and additional component parts are required in such a design.